Signals such as speech are normally transmitted in air by a loudspeaker consisting of a cone mounted on a baffle, the cone being driven by electromagnetic means to create an analog pressure wave corresponding closely to the current variations in the driving motor of the cone. A number of cones may be used to secure more power-radiating capacity, to give a sound source with a large area, and to permit more efficient generation of low-frequency waves.
An alternate type of loudspeaker uses a cone or a diaphragm, driven by electromagnetic means by analog voice currents, which is given an increased aperture by attachment to an exponential horn. In this case the exponent in the mathematical expression for the horn throat area determines the low-frequency cut-off of the loudspeaker, the area of the mouth of the horn determines the apparent area of the sound source, and the directional characteristics of the horn are a functions of both parameters.
Loudspeakers of the types described have low efficiency of sound-power output relative to speech-power input, of the order of 5% to 50%, and have sound-power outputs of a few watts to a few kilowatts. For sound-power output of a kilowatt or more the loudspeakers of the prior art are expensive, heavy and fragile, have limited and irregular frequency response, and require large and expensive sound-power amplifiers to drive them.
Loudspeakers of several types are used under water or other liquids and generally consist of a piezoelectric, magnetostrictive or electromagnetic unit coupled directly to the fluid or driving a diaphragm, and matched in impedance to the body of liquid by means of a horn or similar device. Problems and inadequacies are very similar to those encountered in air loudspeakers, with some additional complications due to the high density of liquids compared to air.
No prior art has been found relevant to this invention, distinguished from earlier transducers by the use of the sampling techniques.